1. Field of the Invention
The invention relates to multicomponent crosslinkable compositions based on silane-crosslinking prepolymers and silicone resins, methods for producing them, and their use as adhesives and sealants, more particularly as adhesives with high tensile shear strength.
2. Description of the Related Art
For adhesive bonds of high tensile shear strength, of the kind employed for numerous wood bonds among others, typically, isocyanate-crosslinking PU adhesives are employed. These PU adhesives usually comprise aromatic polyisocyanates. Systems of this kind cure by reaction of the isocyanate groups with (atmospheric) moisture. Since PU adhesives cure via a chemical crosslinking reaction and are also able to attach chemically to a wood substrate, they exhibit very good mechanical properties and are also comparatively resistant toward external (weathering) effects such as moisture or direct water contact.
The general performance of adhesives is ascertained by their compliance with standards, such as, for example, DIN EN 204, durability class D1-D4. These standards can generally be met by isocyanate-crosslinking adhesives.
Nevertheless, even isocyanate-crosslinking adhesives possess disadvantages inherent in the system such as for example, health-related classification which ranges from sensitizing to toxic. A critical factor is the amount of monomeric isocyanates which remain, and which are removable only with difficulty. This presents problems for the end user, i.e., the craftworker or home improver who comes into contact not only with the fully cured and hence isocyanate-free and entirely unobjectionable product, but also with the isocyanate-containing adhesive or monomeric isocyanates. For the unpracticed home improver there is a particular risk that the products may not be used expertly and/or properly. Additional hazards arise from incorrect storage such as storage within the reach of children. With the professional craftworker, on the other hand, proper use and storage can be assumed. However, a potential problem exists that the professional user is required very regularly—possibly even a number of times a day—to work with the isocyanate-containing material, which is potentially critical in view of the aforementioned sensitizing and also because of possibly carcinogenic effects of isocyanates.
Somewhat more favorable in this respect are isocyanate-crosslinking adhesives which contain only very low levels of volatile monomeric isocyanates and are therefore at least free from labeling requirements. These adhesives too, however, usually contain high concentrations of various oligomeric isocyanates.
An alternative curing technology which is being increasingly applied in the adhesives sector is silane crosslinking where alkoxysilane-functional prepolymers initially undergo hydrolysis on contact with atmospheric moisture and then cure through a condensation reaction. The corresponding silane—functional—usually silane-terminated—prepolymers are entirely unobjectionable from a toxicological standpoint.
Polymer systems which possess reactive alkoxysilyl groups have been known for a long time. On contact with water or atmospheric moisture, these alkoxysilane-terminated polymers are capable of undergoing condensation with one another even at room temperature with elimination of the alkoxy groups. Thus, adhesives based on alkoxysilane-crosslinking polymers exhibit not only good properties of adhesion to the majority of substrates in their fully cured state, but also very good mechanical properties. The adhesives exhibit not only high elasticity but also a tensile strength which is entirely sufficient for many applications.
But a disadvantage of many systems in line with the prior art is the low reactivity of the corresponding MS polymers or SPUR polymers with respect to moisture, necessitating an aggressive catalysis. Therefore, the mixtures in question typically include considerable amounts of toxicologically objectionable tin catalysts.
An advantage is the use of α-silane-terminated prepolymers, which possess reactive alkoxysilyl groups joined via a methylene spacer to an adjacent urethane unit. This class of compounds is highly reactive and requires neither tin catalysts nor strong acids or bases to achieve high cure rates in contact with air.
Another disadvantage of the majority of common silane-crosslinking systems is a relatively low tensile shear strength. Typical applications for this new type of adhesive are confined, consequently, in general to areas in which the requirement is for elastic adhesives more than for adhesives of high tensile strength.
Adhesives with tensile shear strengths >6 MPa, more particularly adhesives with tensile shear strengths >9 MPa, have to this date been producible only using certain highly specific silane-crosslinking systems, inter alia with silane-terminated polyurethanes, of the kind described for example in WO 2011/026658 and its US equivalent US 2012/165493, disclosures of which are incorporated in their entirety by reference herein. The high tensile shear strength is achieved through the use of prepolymers which on the one hand have a very high density of urea units and/or urethane units that are capable of hydrogen bonding and on the other hand are relatively short-chained and thus have a correspondingly high number of crosslinkable silane end groups. Systems of this kind, however, inevitably possess two inherent disadvantages. First, the preparation of prepolymers with a high concentration of silane-crosslinking groups necessitates correspondingly large quantities of silane. These silanes, however, generally constitute the most cost-intensive prepolymer constituents. Secondly, the high concentration of urethane groups and/or urea groups, which is likewise necessary for achieving high tensile shear strengths, leads to very high prepolymer viscosities. Correspondingly problematic are the compounding of these prepolymers to form fully formulated adhesives and the application of these end products, which usually likewise have comparatively high viscosity.